1. Field of the Invention
The present invention relates to a light beam condensing apparatus to introduce a condensing spot of a laser beam onto an optical recording medium such as optical disks, and to a method of driving the optical recording medium by applying the apparatus.
2. Description of the Prior Art
There has been frequently studied a technique to improve recording density of an optical disk so as to provide a large capacity optical disk. Thus, it has been known that reduction of a condensing spot diameter of a laser light beam for recording and regeneration is very effective in the improvement of the recording density, and mean surface recording density substantially increases while being inversely proportional to the square of the condensing spot diameter d.sub.SPOT. The condensing spot diameter d.sub.SPOT is proportional to a wavelength .lambda. of a laser to be used, and is inversely proportional to the numerical aperture NA of an objective lens serving as a condenser, as shown in the following expression (1): EQU d.sub.SPOT =k.multidot.(.lambda./NA) (1)
where the proportional constant k is defined by a wave front distribution of light wave incident on the lens. According to the expression (1), there are available three ways to reduce the condensing spot diameter d.sub.SPOT, i.e., (1) reducing the wavelength of the laser to be used, (2) increasing the numerical aperture of the objective lens serving as the condenser, and (3) using super resolution in a condensing optical system.
A description will now be given of a method for providing a small condensing spot diameter by utilizing the super resolution in the condensing optical system. This method has been often disclosed in articles such as 1) Yamanaka et al., "High Density Recording in Optical Disk by Super Resolution" in Optics, Vol.18, No.12, (1989), or 2) H. Ando, "Phase-Shifting Apodizer of Three or More Portions" in Japanese Journal of Applied Physics, Vol.30, (1991). In these methods disclosed in the articles, it is possible to reduce the condensing spot diameter on the basis of the same principle, as shown in FIGS. 1 and 2.
FIG. 1 shows a configuration of an optical system of a conventional super resolution optical head as an example. In FIG. 1, reference numeral 101 means a laser oscillator, 102 means a collimate lens, 103 is a beam forming prism, 105 is an objective lens, 106 is a recording medium, and 107 is a shading plate.
A description will now be given of the operation. Laser light from the laser oscillator 101 serving as a light source is collimated through the collimate lens 102 and the beam forming prism 103, resulting in parallel light. A laser beam 104 serving as the parallel light is focused and condensed by the objective lens 105 on a recording surface of the recording medium 106. Here, the shading plate 107 is disposed across the laser beam 104 so as to partially shade the laser beam 104. At the time, the condensing spot diameter d.sub.SPOT of the laser beam 104 is varied according to a position and a shape of the shading plate 107, that is, a width and a length thereof.
A description will now be given of the principle in the reduction of the condensing spot diameter by the super resolution with reference to FIG. 2. As shown in FIG. 2, in case the shading plate 107 is longer than a beam diameter D of the collimate beam 104, a condensing spot diameter d.sub.SPOT t in a cross direction of the shading plate is defined as a ratio of the beam diameter D to the width .DELTA.W of the shading plate 107 if the width of the shading plate 107 is defined as .DELTA.W. Further, a condensing spot diameter d.sub.SPOT r in a longitudinal direction of the shading plate 107 is substantially irrelevant to the width .DELTA.W. Here, as the width .DELTA.W becomes larger, sidelobes 108 in a condensing spot becomes higher while the condensing spot diameter d.sub.SPOT t of a mainlobe 109 becomes smaller.
FIG. 3 shows a relation between .DELTA.W/D and the condensing spot d.sub.SPOT t. As understood from FIG. 3, as .DELTA.W/D is more increased, the condensing spot diameter d.sub.SPOT t is reduced, and concurrently the intensity of the sidelobe is an increased. Since, increase of the sidelobe intensity causes an increase of crosstalk, it is impossible to allow the sidelobe to become so large. Here, .DELTA.W/D=0 if the shading plate 107 is not employed. At the time, if the condensing spot diameter is set to d.sub.SPOT 0, it is possible to reduce the condensing spot diameter d.sub.SPOT t to 10% degree as compared with d.sub.SPOT 0 when the sidelobe intensity can be in a range of 0.1 times the mainlobe or less. In such a way, it is possible to reduce the condensing spot diameter with the constant laser wavelength .lambda. and the constant numerical aperture NA of the lens by shading a vicinity of an intermediate portion of the collimate beam in a super-resolution optical head. When the shading plate 107 is coplanarly rotated by 90.degree., the condensing spot diameter d.sub.SPOT t is left as it is d.sub.SPOT 0, and the condensing spot diameter d.sub.SPOT r is reduced.
As set forth above, the principle of the super resolution utilizes the character of focusing light wave that it is possible to vary the intensity distribution at the condensing spot by modulating a wave front of the collimate beam 104 on an entrance surface of the objective lens 105. That is, the shading plate 107 shown in FIG. 2 corresponds to space modulation which is performed so as to set an amplitude distribution of the collimate beam 104 on the entrance surface of the objective lens 105 to zero in the vicinity of the intermediate portion of the collimate beam 104. Accordingly, laser power at a shaded portion is lost.
Further, on the basis of the principle of the super resolution, it is also possible to vary the intensity distribution of the condensing spot by modulating a phase distribution of the collimate beam 104 on the entrance surface of the objective lens 105. That is, it is possible to form a condensing spot shape by providing appropriate phase shift according to a position on the entrance surface of the objective lens 105. This method is employed in the article 2) as described before. In this case, the collimate beam 104 is not shaded so that there is no partial loss of the laser power due to the shading.
Alternatively, in another known technique, a distribution is caused in indexes of refraction in order to provide phase modulation to transmitted light. Assuming that there is a difference .DELTA.n between the indexes of refraction sensed by the transmitted light at two portions of a modulation plate when light having the wavelength .lambda. passes through the modulation plate having a thickness of L. Consequently, in the light beam passing through both portions, there is generated a phase difference .DELTA..phi. expressed by the following expression (2): EQU .DELTA..phi.=2.pi.(L/.lambda.).multidot..DELTA.n (2)
A phase of the transmitted light is modulated by the phase difference. It must be noted that a method of the phase modulation of the transmitted light should not be limited to a method to provide a difference in an optical path length by the difference in the indexes of refraction. It is similarly possible to provide the difference in the optical path length by varying the thickness of the modulation plate so as to perform the phase modulation of the transmitted light.
The recording density of the optical disk can be expressed by the product of the recording density in a direction parallel to a recording track (i.e., track recording density BPI) and the recording density in a direction perpendicular to the recording track (i.e., track density TPI). Therefore, it is possible to improve surface recording density of the optical disk by improving the BPI and the TPI, respectively. The conventional embodiment shown in FIG. 2 is provided to improve the BPI. For example, if a concentrically circular shading plate to shade the intermediate portion exclusively is employed instead of the shading plate 107 shown in FIG. 2, the condensing spot has a concentrically circular shape so that the mainlobe 109 is surrounded by the sidelobe 108. In this case, the condensing spot diameter of the mainlobe 109 can be reduced. Thus, it is possible to concurrently improve the BPI and the TPI by using the condensing spot.
As set forth above, the reduction of the condensing spot diameter by the super resolution is an effective technique to improve the recording density. According to the prior art, it is possible to provide a constant condensing spot diameter by varying the amplitude or the phase of the transmitted light by a fixed optical component such as the shading plate, or the phase plate. However, in the prior art, it is impossible to vary a parameter of the super resolution, that is, the modulation amount applied to the wave front of the collimate beam on the entrance surface of the objective lens during the operation in one condensing apparatus so as to dynamically vary the condensing spot diameter or the condensing spot shape of an optical disk unit.
On the other hand, in the current market, there are employed optical disks compatible to the optical disk standard which is standardized by the ISO standard or the like. Most of these disks have a track pitch of 1.6 (.mu.m) and a track recording density of 25 (kbit/inch). Further, a large capacity optical disk has been developed in recent years, and the track pitch is reduced and the track recording density is increased if it is possible to provide the practical large capacity optical disk having an improved recording density than that of the conventional optical disk. Accordingly, a condensing spot diameter smaller than that in the prior art is required for recording and regenerating information. The condensing spot diameter can be provided by applying a shorter wavelength laser, an objective lens having a larger numerical aperture, or the super resolution as described before. In this case, it is to be understood that the condensing spot diameter is designed so as to be adaptable to the track pitch or the track recording density of a newly developed large capacity optical disk.
Here, compatibility of an optical disk drive becomes a major issue. That is, in case the optical disk drive is provided with a function to drive both the newly developed large capacity optical disk and the optical disks based upon the conventional standard, there are the following three problems. The first problem relates to a tracking servo. A servo sensor signal for tracking is detected depending upon diffraction phenomena of the spot on the disk surface because of a guide groove, i.e., a periodic structure of a groove and a land on the optical disk. Hence, if the condensing spot is designed so as to be adaptable to a narrow-width track pitch, there is a drawback in that it is not possible to sufficiently provide a servo error signal for tracking when the conventional optical disk having a wide-width track pitch is driven.
The second problem occurs at a time to read an emboss signal. While information of the emboss signal is recorded on the optical disk in a form of a phase bit, the signal regeneration is performed depending upon a principle that condensed light is diffracted by the phase bit, and an amount of reflected light to be received by the detector is varied according to the presence or absence of the bit. Therefore, it is impossible to provide a sufficient variation rate by the diffraction in case the condensing spot diameter is too small with respect to the phase bit. As a result, there is another drawback in that reading accuracy is reduced or incapability of reading occurs due to a reduced regenerative amplitude of the emboss signal.
The third problem occurs when the information on the medium is erased in a rewritable optical disk. In optical disks which are recorded and erased by thermal energy of the condensed light such as magneto-optical medium, or phase varying medium, when a signal recorded on a low density medium having the wide-width track is erased by the condensing spot having a small diameter, it is impossible to erase an entire width of the recorded mark since an erasable width is narrow, resulting in an unerased portion. As a result, there is still another drawback in that the unerased portion is left as the crosstalk, and increases the occurrence of regeneration error when the erasing and recording is repeated.